Despite sensitive radioimmunoassay tests for HBsAg applied to all donor blood, some type B hepatitis continues to occur following transfusion. It has previously been suggested that HBsAg negative individuals who have anti-core antibody may transmit the hepatitis B virus. The availability of a new sensitive radioimmunoassay for anti-core antibody allows this postulate to be tested in our prospectively followed, open heart surgery patients. Similarly, we can ascertain if chronic carriers of non-A, non-B hepatitis have elevated serum transaminase; and, if so, whether this could be employed to screen donors and reduce the frequency of post-transfusion hepatitis due to this agent.